1. Field of the Invention
This invention relates generally to non-destructive methods of detecting and evaluating metallic corrosion. This invention relates more specifically to non-destructive methods for rapidly detecting and locating corrosion of reinforcing steel that occurs within concrete structures, by analyzing the non-linear harmonic and intermodulated frequencies of transmitted electromagnetic waves.
2. Description of Related Art
The utilization of concrete as a structure supporting and load bearing material has long been supplemented by the inclusion of reinforcing steel within the concrete. One of the unfortunate consequences of this inclusion, however, is the inevitable deterioration of the reinforcing steel when exposed to an intrusion of water, salt, oxygen, and other minerals that initiate chemical reactions. Not only can this deterioration of the reinforcing steel weaken its properties, it can exacerbate the deterioration of the concrete itself.
Reinforced concrete is utilized in numerous structures, including buildings, bridges, and other highway features. For this reason, it is important that engineers be able to determine the condition of reinforcing steel within concrete structures so as to assure their safety and reliability. It is also important to be able to obtain information on the integrity of reinforcing steel without the necessity of exposing it to visual inspection.
Presently, electrical potential measurements are widely used for monitoring the corrosive state of steel reinforcing bars in concrete. With these methods, the electrical potential between the steel reinforcement and a reference electrode, which is in physical contact with the surface of the concrete (if the concrete is dry, it must first be moistened), is measured using a voltmeter. These methods detect the presence of corrosive activity, but not the extent and location of corrosion damage. To make electrical potential measurements, a direct electrical connection must be made to the reinforcing steel. Therefore, if the steel is not exposed, some concrete covering must be removed.
Other methods that have been attempted include imbedding probes within the concrete structure adjacent to the reinforcing steel members so as to provide a means for returning at a later date, and making measurements with the probe to determine the rate or stage of deterioration. U.S. Pat. No. 4,703,255 and U.S. Pat. No. 4,703,253, both issued to strommen, describe particular embodiments of such a corrosion probe. Unfortunately, these probes must be installed at the time the concrete is poured in order to anticipate their use at a later date, and are useful only for that localized area.
There are many patented methods for utilizing electromagnetic waves, particularly in the radar frequencies, to detect and locate objects either under the ground or within concrete structures Some of these methods are disclosed in the following patents:
______________________________________ Number Patentee Date Title ______________________________________ 4,072,942 Alongi 02/07/78 Apparatus for the Detection of Buried Objects 4,691,204 Hiramoto 09/01/87 Radar Apparatus 4,698,634 Alongi, et al 10/06/87 Subsurface Inspection Radar 4,706,031 Michiguchi, 11/10/87 Method and System for et al Detecting an Object with a Radio Wave 4,839,654 Ito, et al 06/13/89 System for Detecting Underground Objects ______________________________________
The utilization of radar, and of any other electromagnetic radiation for that matter, has generally been limited to the detection and location of reinforcing components within concrete structures, rather than an actual analysis of the stage of deterioration these reinforcing components are in.
Metal-to-metal junctions that are separated by thin, non-metallic materials such as oxides, are known to exhibit non-linear electrical behaviors Examples of these kinds of junctions include metal-to-metal contacts, corrosion, stress corrosion cracks, and fatigue cracks. Because of the antisymmetry of the voltage-current curve associated with these junctions, the non-linear effects produced are odd numbered with the third order effects being predominant. This phenomenon is well known in the fields of electronics and telecommunications, since it is a source of undesired noise.
When metal-to-metal junctions are excited at two frequencies, harmonics (HM) and intermodulation (IM) frequencies are generated due to the non-linear effects of the junction. For example, when these junctions are excited at frequencies f.sub.1, and f.sub.2, the HM and IM signals generated include 3f.sub.1, 3f.sub.2, f.sub.1 .+-.2f.sub.2, 2f.sub.1 .+-.f.sub.2, etc. The corrosion of such junctions significantly enhances this effect and thus the noise problem associated with these junctions. Both the HM and IM signals increase in amplitude as the degree of corrosion increases.
The utilization of these HM and IM signals that are generated by corroded contacts has been limited primarily to their detection and elimination from electronic equipment, where their presence adds to the background noise levels.